1. Field of the Invention
This invention relates to a method and an apparatus for detecting the leading edge of a sheet-like object which is to be scanned in an image scanning system. More particularly, this invention relates a method and an apparatus for accurately detecting the leading edge of an image bearing sheet to be scanned in an image scanning system in which main scanning is performed on the sheet by making a beam of light oscillate back and forth quickly across the sheet in one direction and sub-scanning is performed on the sheet by moving the sheet in the direction perpendicular to said one direction.
2. Description of the Prior Art
In an image scanning system in which main scanning is performed on a sheet bearing image information by making a beam of light oscillate back and forth quickly across the sheet in one direction and in which sub-scanning is performed on the sheet by moving the sheet in the direction perpendicular to said one direction, it is not always necessary to detect the leading edge of the sheet which is on the way to the scanning station.
Sometimes, however, superposition processing is required in which a plurality of stimulable phosphor sheets or X-ray sensitive films each bearing the image or picture of the object simultaneously or sequentially taken from the same direction are scanned one by one to obtain the corresponding plurality of image signals, which are later superposed on each other to obtain an image or picture of high quality which is characterized by high signal-to-noise ratio and high contrast. Sometimes, a subtraction processing is required in which a selected portion is removed from an image or picture. The superposition and subtraction processings are described in detail in Japanese Unexamined Patent Publication No. 56(1981)-11400 and U.S. patent application Ser. Nos. 168,800 and 168,802. In these image processings, however, every time a subsequent sheet is presented to the scanning station, it is necessary for the scanning to start from the same point on the sheet thus presented. This necessitates the exact detection of the leading edge of each sheet on arrival at the scanning station.
For detecting the leading edge of each sheet to be scanned there have been hitherto proposed a mechanical detection system using limit switches, an optical detection system using a light projector and a light acceptor combined in the form of a photosensor, and an electric system using, in combination a timer and one of the aforesaid mechanical or optical detection systems, thereby detecting the leading edge of the sheet on its way to the scanning station with the aid of the mechanical or optical detection system, and then measuring a predetermined length of time with the aid of the timer so as to predict the arrival of the leading edge of the sheet at a desired place after the lapse of the predetermined length of time from the detection of the leading edge of the sheet.
There are, however, various problems in practical use of these conventional detecting systems as mentioned below, and what is worse is that they are unable to detect the leading edge of the sheet with accuracy.
In the mechanical detection system using limit switches, there is little or no degree of freedom in positioning the limit switches, and accordingly there are strict restrictions on positioning the detection system in the image scanner. Also, the accuracy in detection is relatively low, and the detecting position cannot, in fact, be changed, and therefore in handling sheets of different sizes it is necessary that there are provided a plurality of limit switches each allotted to the sheets of a different size. Detection is performed at a single point on the leading edge of the sheet, and therefore if there is a recess or irregularity on the leading edge of the sheet, the recess or irregularity causes erroneous detection. A mechanical force comes to bear on the sheet when it is brought in contact with the limit switch, and therefore it is possible that the sheet may be caused deviate from its course, thus making the feeding of sheets unreliable.
The optical detecting system using photosensors requires exact alignment of a light projector and a light acceptor at the detecting position, and accordingly there are strict restrictions on selecting positions. The detecting position cannot be changed, and detection of only a single point on the sheet causes appreciable errors. Different from the mechanical detection system, however, the optical detection system causes no mechanical force to bear on the sheet, and therefore the sheet can be advantageously fed in a stable and reliable condition. However, the optical detecting system requires that a light projector and a light acceptor be provided on opposite surface sides of the sheet, and this disadvantageously increases the cost and labor required for fabricating the system. The optical detection system is able to detect the leading edge of the sheet with higher accuracy than the mechanical detection system using limit switches. There is, however, a certain limit on the possible degree of reduction of the size of the beam spot or of the light receiving area of the light acceptor, and therefore it is not possible to obtain sufficient resolution to realize the required precision in image-superposition.
In the case of limit switches and photosensors, the position at which these units are fixed and the position at which detection should be performed are inevitably a small distance apart from each other, and therefore their positioning requires delicate positional adjustment.